1. Field of the Invention
The present invention relates to a luminescent screen comprising a resonant microcavity having a phosphor active region.
2. Description of the Prior Art
Conventional cathode ray tube (CRT) displays work by projecting electrons from an electron gun, which accelerates them by passing them through an intense electrical field, onto a screen coated with a phosphor material in the form of a powder. The high-energy electrons excite luminescence centers in the phosphors which emit visible light uniformly in all directions. CRT's are well established in the prior art and are commonly found in television picture tubes, computer monitors and many other devices.
Displays using powder phosphors suffer from several significant limitations, including: low directional luminosity (i.e., brightness in one direction) relative to the power consumed; poor heat transfer and dissipation characteristics; and a limited selection of phosphor chromaticities (i.e., the colors of the light emanating from the excited phosphors).
The directional luminosity is an important feature of a display because its directional properties influence the efficiency with which it can be effectively coupled to other devices (e.g., lenses for projection CRT's). The normal light flux pattern observed from a luminescent screen closely follows a "Lambertian distribution"; i.e., light is emitted uniformly in all directions. For direct viewing purposes this is desirable, as the picture can be seen from all viewing angles. However, for certain applications a Lambertian distribution of the light flux is inefficient. These applications include projection displays and the transferring of images to detectors for subsequent image processing.
Heat transfer and dissipation characteristics are important because one of the limiting factors in obtaining bright CRT's suitable for large screen projection is the heating of the phosphor screen. As the incident electron beam density increases, the phosphor temperature increases. When the phosphor reaches a certain temperature, its luminosity decreases. This is known as thermal quenching. With conventional powder-phosphor displays the phosphor-to-screen heat transfer characteristics are relatively poor, therefore heat dissipation is limited and thermal quenching can occur at relatively low beam densities. Because projection displays require high beam densities to produce the brightness required to project an image, this inefficiency makes conventional CRT's poorly suited for projection displays.
Chromaticity is important because the faithful reproduction of colors in a display requires that the three primary-color phosphors (red, green and blue) conform to industry chromaticity standards (e.g., European Broadcasting Union specifications). Finding phosphors for each of the three primary colors that exactly match these specifications is one of the most troublesome aspects of phosphor development.
The decay time of the activator (i.e., light emitting ion in the phosphor) is also another important parameter for a phosphor. In an ideal phosphor for high brightness applications, it is desirable to control directly the decay time of the phosphor for each display application. This allows rapid re-excitation of the activator with a corresponding increase in the maximum light output. The decay time is given by the natural spontaneous transition rate of the activator. In order to improve phosphor performance it is therefore desirable to have control over this spontaneous transition rate.
Another problem encountered in conventional phosphor displays is that energy can transfer from one activator to another nearby activator in the phosphor host matrix. This is a nonradiative process where the efficiency of the phosphor is reduced. The energy transfer is strongly dependent on activator concentration and therefore it limits the density of activators that can be incorporated in a display and thus the maximum light output.
The use of a single-crystal, thin-film phosphor as a faceplate for a CRT was first described in a British patent application by M. W. Van Tol, et al., UK Pat. GB-2000173A (1980). This patent taught the use of an yttrium aluminum garnet Y.sub.3 Al.sub.5 O.sub.12 (YAG) film grown by liquid phase epitaxy (LPE) ona single-crystal YAG substrate. The YAG film is doped with a rare-earth ion which emits light when excited by electrons. (Doping is the process wherein dopant ions are substituted for host ions in the crystal lattice during crystal growth.) In this device, the thickness of the thin-film layer is from one to six microns and does not bear any relation to the wavelength of the light to be emitted by the display.
This device exhibited several advantages over conventional powder-phosphor displays. One such advantage was that heat was transferred from the phosphor more efficiently because of the perfect contact between the phosphor and the screen, and because of the high thermal conductivity of the YAG substrate. The screen could be loaded with a higher beam density without exhibiting thermal quenching and, therefore, could produce more light.
Another advantage of single-crystal phosphor luminescent screens versus powder deposited luminescent screens is concerned with the resolution of a pixel (i,e., light producing spot). For high resolution displays using powder phosphor, the limiting size of a pixel--and hence the resolution of the screen--is determined by the particle size of the phosphor powder. Single-crystal phosphors, on the other hand, are not affected by this since they do not contain discrete particles, but have a homogeneous distribution of phosphor ions substituted in the host lattice instead.
Powder phosphors further reduce resolution due to the light scattering from the surface of the powder. Because of the lack of discrete phosphor particles and the absence of light scattering, thin-film displays have high image resolution, limited only by the spot size of the exciting electron beam. The increasing demand for higher resolution displays makes this a particularly attractive advantage.
Yet another advantage is concerned with producing a vacuum in a CRT. To allow the electron beam to travel between the electron gun and the phosphor screen, a vacuum must be maintained within a CRT. Conventional powder phosphors have a high total surface area and, generally, organic compounds are used in their deposition. Both the high surface area and the presence of residual organic compounds cause problems in holding and maintaining a good vacuum in the CRT. Using thin-film phosphors overcomes both of these effects, as the total external surface area of the tube is controlled by the area of the thin-film (which is much less than the surface area of a powder phosphor display) and, furthermore, there are no residual organic compounds present in thin-film displays to reduce the vacuum in the sealed tube.
The thin-film phosphors of Van Tol, et al., exhibit one prohibiting disadvantage, however, due to the phenomenon of "light piping." Light piping is the trapping of light within the thin-film, rendering it incapable of being emitted from the device. This is caused by the total internal reflection of the light rays generated within the thin-film. Since the index of refraction (n) of most phosphors is around n=2, only those light rays whose incident angles are less than the critical angle, .theta..sub.c (where sin .theta..sub.c =1/n) will be emitted from the front of the thin-film. The critical angle for an n=2 material is around 30.degree.. Therefore, the fraction of light that escapes from the front of the thin-film is only about 6.7% of the total light. The common design of placing a highly reflective aluminum layer behind the film only doubles the output to about 13% of the light. Moreover, this light is spread in a "Lambertian distribution" and is not directional. As a result of light piping, the external efficiency (i.e., the percentage of photons escaping from the display relative to all photons created in the display) is less than one-tenth that of powder phosphor displays. Therefore, in spite of the unique advantages offered in terms of thermal properties, resolution, and vacuum maintenance; the development of commercial CRT devices based on thin-films is held back by their poor efficiency due to "light piping".
Some schemes have been designed to reduce the "light piping" problem. One scheme described by Bongers, et al., U.S. Pat. No. 4,298,820 (1981), uses a thin-film, deposited by LPE, with V-shaped grooves etched into the surface to reflect light out of the thin-film. This approach brought about an improvement in external efficiency of around 1 1/2 to 2 1/2 times that of a thin-film display without the V-shaped grooves. Given the previous external efficiency of 13%, this would still only lead to a total external efficiency of around 20% to 30%.
Another scheme, described by Huo and Hou, "Reticulated Single-Crystal Luminescent Screen", 133 J. Electrochem. Soc. 1492 (1986), involves etching individual mesa shapes onto the thin-film deposited by LPE. This led to a three times improvement in external efficiency (still rendering only about a 30% external efficiency). Furthermore, since the phosphor layer was no longer, strictly speaking, a thin-film, any light rays that were internally reflected could find themselves rescattered to areas far from their point of creation, thus spoiling the resolution of the display.
Microcavity resonators, which are incorporated in the present invention, have existed for some time and have recently been described by H. Yokoyama, "Physics and Device Applications of Optical Microcavities" 256 Science 66 (1992). Microcavities are devices which have the ability to control the decay rate, the directional characteristics and the frequency characteristics of luminescence centers located within them. The changes in the optical behavior of the luminescence centers involve modification of the fundamental mechanisms of spontaneous and stimulated emission. Physically, microcavities are optical resonant cavities with dimensions ranging from less than one wavelength of light up to tens of wavelengths. These are typically formed as one integrated structure using thin-film technology. Microcavities involving planar, as well as hemispherical, reflectors have been constructed for laser applications.
Resonant-microcavities with semiconductor active layers, for example silicon or GaAs, have been developed as semiconductor lasers and as light-emitting diodes (LEDs).
E. F. Schubert, et al., "Giant Enhancement of Luminescence Intensity in Er-doped Si/SiO2 Resonant Microcavities" 61(12) Appl. Phys. Lett. 1381 (1992), describes a resonant microcavity with an Er doped SiO.sub.2 active layer. This device emits radiation in the infrared region and is intended as a laser amplifier for fiber-optic communications.
The Schubert device, the semiconductor lasers and the LEDs are unsuitable for use in luminescent displays for several reasons. They contain luminescent materials such as Si, GaAs,.etc., in the active region which are suitable as laser media, but which are inefficient emitters of visible light. They also are designed with small planar surface areas that are inadequate for display purposes. Moreover, because of the design of these devices and the active materials used, they cannot be excited efficiently with electron bombardment, an electric field, or ultraviolet radiation. These excitation mechanisms are an essential part of the current display technologies.
Furthermore, the laser microcavity devices work above the laser threshold, which means that their response to excitation is inherently nonlinear and their brightness is limited to a narrow dynamic range. Displays, conversely, require a wide dynamic range of brightness. Microcavity lasers are also unsuitable for use in displays because the laser light they produce is highly coherent. Highly coherent light exhibits a phenomenon called speckle. When viewed by the eye, highly coherent light appears as a pattern of alternating bright and dark regions of various sizes. To produce clear, images, luminescent displays must produce incoherent light.